fix bad interaction between nickTimer.Stop() and accounts.Logout()

Sequence of events:

1. client.nickTimer.Stop()
2. client.server.accounts.Logout(client)
3. accounts sees that client is no longer logged in, does client.nickTimer.Touch()
4. 30 seconds later, RandomlyRename resurrects the zombie client

irc/idletimer.go

 	client  *Client
 
 	// mutable
+	stopped        bool
 	nick           string
 	accountForNick string
 	account        string
 	func() {
 		nt.Lock()
 		defer nt.Unlock()
+
+		if nt.stopped {
+			return
+		}
+
 		// the timer will not reset as long as the squatter is targeting the same account
 		accountChanged := accountForNick != nt.accountForNick
 		// change state
 		nt.timer.Stop()
 		nt.timer = nil
 	}
+	nt.stopped = true
 }
 
 func (nt *NickTimer) sendWarning() {